Optical transmission systems have enjoyed increasing usage throughout telephone network and switching systems. For example, optical fiber systems have replaced copper based transmission systems in many trunk and junction networks. As optical fiber systems become more pervasive in the user area of the telecommunication system, better and less expensive usage of the capabilities of optical fiber transmission systems will be in demand.
Within the user area, a contemplated application of optical transmission systems is for connection to a broadband integrated services digital network ("ISDN") for providing interactive services including the distribution of video signals in the local loop. The optical transmission system configuration envisioned for such a connection network is a shared medium passive optical network ("PON") having a tree-and-branch topology. This topology provides a cost-effective way to connect small businesses and residential subscribers to an ISDN, and has been used in several field trials for Plain Old Telephony Services and distributive services, such as the Amsterdam-Sloten Fibre to the Home Field which is described in H. H. Grotjohann, F. Jaeger and P. E. Schaafsma, Dutch FTTH on Trial, Communications International, p. 49 (January 1992).
In a PON, the optical fiber is shared by a group of customers. Further, in a passive optical network having a tree-and-branch topology, a single fiber emanates from a local exchange and fans out via passive optical splitters and tree couplers to a plurality of service customers.
One method for transmitting information from the local exchange to individual customers using such a topology is as follows. Information is transmitted in packets or cells. Each transmitted cell contains a routing address identifying the intended destination customer. All information cells are transmitted through the entire PON to optical receivers located at the ends of the network which are called optical network terminations ("NT"). Each NT is connected to a corresponding group of customers. When an NT receives an information cell, it compares its address with the routing address contained within the cell. When the addresses match, the information is delivered to the customer. When a match is not detected, the NT discards the received information and no transmission is made to any associated customers.
A potential problem of eavesdropping exists in such a network because an unscrupulous customer would have access to all the information transmitted by the local exchange if he were to tap into his NT. Several measures have been implemented to add some level of security and privacy to shared medium systems. Some of these measures include various forms of encrypting the transmitted information. In such systems, it is intended that only the destination NT be able to decrypt the information cell and deliver the decrypted original information to the intended subscriber.
One conventional method of encryption is to combine a pseudo-random stream of binary bits to the information cell to be transmitted using modulo 2 addition. Generation of the pseudo-random sequence can be accomplished by dedicated hardware, such as feedback shift registers, or in software. Deciphering the encrypted information stream occurs by combining, using modulo 2 addition, the encrypted information stream with the identical pseudo-random sequence used for encryption.
In addition to encryption for security purposes, it is also desirable to scramble or rearrange the bits of the information stream to avoid transmitting periods of DC signals which occurs when continuous sequences of either logic ones or zeros are transmitted. Scrambling is performed on a transmitted signal, in order to increase the number of 0-to-1 transitions, thereby minimizing the DC content. DC signals interfere with clock recovery in the network termination. Clock recovery is the generation of a local clock signal that is derived from the received information to permit synchronous, in phase, reading of the transmitted information.
In conventional information transmission systems, the operations of encryption and scrambling have been performed by separate hardware. Likewise, the operations of descrambling and decryption have also been performed using separate hardware.
A reduction in hardware for the operations of scrambling and encryption would result in a less complex system and a cost savings for each of the local exchanges. Likewise, a reduction in the hardware for the operations of descrambling and decryption would result in a similar reduction in complexity and cost savings in each of the NTs. However, a cost reduction in the NTs results in a substantial savings because of the very large number of NTs employed.